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We give an idea and the order-of-magnitude estimations to explain therecently reported secular increase of the Astronomical Unit (AU) by Krasinskyand Brumberg (2004). The idea proposed is analogous to the tidal accelerationin the Earth-Moon system, which is based on the conservation of the totalangular momentum and we apply this scenario to the Sun-planets system. Assumingthe existence of some tidal interactions that transfer the rotational angularmomentum of the Sun and using reported value of the positive secular trend inthe astronomical unit, $\frac{d}{dt}{AU} = 15 \pm 4 {(m/cy)}$, the suggestedchange in the period of rotation of the Sun is about $21 {ms/cy}$ in the casethat the orbits of the eight planets have the same "expansion rate." This valueis sufficiently small, and at present it seems there are no observational datawhich exclude this possibility. Effects of the change in the Sun's moment ofinertia is also investigated. It is pointed out that the change in the momentof inertia due to the radiative mass loss by the Sun may be responsible for thesecular increase of AU, if the orbital "expansion" is happening only in theinner planets system. Although the existence of some tidal interactions isassumed between the Sun and planets, concrete mechanisms of the angularmomentum transfer are not discussed in this paper, which remain to be done asfuture investigations.

Secular increase of the Astronomical Unit: a possible explanation in terms of the total angular momentum conservation law